1. Field of the Invention
The invention, in general, relates to a novel method of controlling the movement of a laser engraving or printing head and, more particularly, to a method of controlling energization and discharge of a laser relative to its movement over a work surface to produce therein an undistorted image based on a bitmap.
2. Background of the Invention.
Apparatus utilizing a laser for engraving, or at least writing on, a suitable surface are well known. An example of such an apparatus is disclosed by U.S. Patent (Application No.: 09/508,739 filed Mar. 15, 2000) by Guettler and Penz. The apparatus functions by a tool head supporting a lens connected to a laser being moved relative to a work piece which is supported on a work surface and by periodically aiming pulses of collimated coherent light at the work piece to affect therein an image-wise surface alteration, by a plurality of indentations or pixels selectively placed so that together they form an image. The image may or may not be an inversion of a scanned or stored positive pattern or bitmap. The movement of the tool head may be responsive to signals, either directly or by way of a storage, derived from a device which mechanically or optically scans the pattern. The work piece may, for instance, consist of coated or anodized metals, paper, wood, or rubber etc. which is susceptible to the formation of indicia therein as a result of laser beam treatment.
The number of coherent light or laser beam discharges per inch of displacement of the laser tool head over the work piece and the duration of each pulse may be varied as an incremental position voltage as a function of the square root of the sum of the squares of the components of displacement of the tool head in orthogonal directions.
Basically, laser printing or engraving is carried out by line-scanning a bitmap and focussing a laser beam on a work piece, the laser beam being switched on at every image point (pixel) or off at every blank position, as the case may be, to form what for the sake of convenience may be referred to as a black and white image in the work piece. Gray-scale images are generated by changes in the intensity of the laser beam by modulating its pulse width. An encoder connected to the drive of the laser tool head provides position signals (pulses per angular unit) to a processor which in turn energizes the laser as a function of the pulses. Ideally, the laser is energized on the basis of a signal representative of the actual position of the tool head. The effect of any lag error, i.e. the difference between the desired and the actual position of the laser along a predetermined curve or line, may be insignificant and tolerable.
In processes of engraving rubber for fabricating stamps, a laser tool head may be moved at a velocity of about 30 cm/sec, and the maximum laser output power may not exceed 50 W. For engraving plastic tags or anodized aluminum, for example, the power output of lasers can generally be kept low as minimum power levels are required for the evaporation of anodized or plastic layers. Hence, the laser tool head may be moved at high velocities. In fact, apparatus is known wherein the laser tool head is moved at a velocity exceeding 1 m/sec. The quality of the resultant engravings have, however, been found to be wanting because of what appears to be an insufficient correction of the laser energization or compensation of the head velocity.
The problems inherent in bidirectional engraving processes in particular, are unfortunately readily apparent since pixels are directionally displaced, i.e. they are not aligned in precise vertical columns. In bidirectional engraving processes, the large time constant of the pulsed laser, at high velocities of the movement of the laser tool head, does in fact yield unusable results.
The difficulties inherent in energizing a laser render it difficult to provide high-speed engraving processes of acceptable precision with cost-efficient x-y plotters. The laser may be discharged at a rate of, e.g. of 500 ppi to form 500 pixels per linear inch of surface. In order to obtain an engraving of acceptable quality, the lag in energizing the laser as results from the physically and structurally conditioned response rate has to be adjusted as a function of the laser head velocity relative to the degree of plasma pre-ionization. Unless the lag is adjusted appropriately, a velocity of 1.5 m/sec of head movement and a laser energization period of  greater than 100 xcexcsec lead to unacceptable errors greater than +/xe2x88x92150 xcexcm. This is thrice the empirical resolution of 50 xcexcm of the human eye.
It is a primary object of the invention to provide a method of coordinating the intermittent movement of a tool head and the periodic discharge of a laser relative to a signal derived from a scanned pattern or bitmap such that pixels are formed in a work piece so as to result in a distortion-free image.
A more general object resides in a method of line engraving on the basis of a bitmap from a suitable source.
A particular object of the invention is to provide a corrective algorithm for controlling the interaction between the high-velocity intermittent movement of a laser head and the related periodic discharge of the laser to yield engravings of superior quality.
Still another object of the invention is to provide a method of bidirectionally engraving pixels in the surface of a work piece by a laser such that each individual pixel of a pattern is shifted to compensate for the lag of response of the laser.
It is also an object of the invention to correct image points as a function of the instantaneous velocity of the movement of the laser head in accordance with a predetermined velocity profile.
Yet another object of the invention resides in compensating for the inherent lag of laser energization by tool head movement.
A still further object of the invention resides in a method of laser engraving wherein a laser is discharged as a function of a correctional laser head displacement signal.
Other objects will in part be obvious and will in part appear hereinafter.
In accordance with the above and other objects, the invention, in a preferred embodiment thereof, provides for shifting each pixel of an image so as to compensate for the lag in the discharge of the laser either on-line by a corrective process as a function of the instantaneous velocity or off-line on the basis of a preprogrammed velocity profile.
Preferably, the correction or compensation takes place during the acceleration path on the basis of an acceleration profile stored in the memory of a control device or on the basis of acceleration data collected by an encoder and fed to a control device.